It is particularly important in gimbal-mounted infrared sensor systems typical of, for example, missile born sensors that the system be extremely compact. In addition, it is important in such infrared systems to avoid field flatteners or separate lenses placed near the image for controlling field curvature; the infrared detector sees itself in any such lens and causes a cold spot in the system output because the detector area is cold. In addition, it is important that the system have a weight distribution providing a relatively small moment of inertia to minimize power requirements for rotating the system by the gimbals.
These requirements have been found to be particularly well satisfied by a concave primary mirror and a convex secondary mirror comprising a configuration often referred to as a cassegrainian arrangement. (The precise definition of a Cassegrain system is one where the primary mirror is specifically a parabola and the secondary mirror a hyperbola. However, systems comprising a concave primary mirror and a convex secondary mirror are now often referred to as cassegrainian systems without particular reference to the particular geometry of the mirors.) Two-mirror arrangements of this kind offer the maximum ratio of focal length to system length.
The compactness of such a configuration is improved as the primary mirror focal length is shortened requiring, as a consequence, a related decrease in the focal length of the secondary mirror. As this design feature is extended, such a system normally shows an increasing amount of field curvature arising from the disparity in the magnitude of radii of the primary and secondary mirrors.
In prior art systems, control of field curvature has typically been handled through use of field flatteners or additional lenses located within the system. See for example, U.S. Pat. No. 3,515,461, Casas et al, June 2, 1970, column 2, lines 44-46. As previously mentioned, if such a field flattener or corrector is located near the image in an infrared system, it has a distinct disadvantage since the detector can see itself as a reflection in the correcting lens. In addition, systems having such additional lenses are heavier and more complex.
In systems where these factors are a problem, a particular feature of the present invention is the use of a mangin mirror having its surfaces shaped for providing control of field curvature without the use of special additional lenses.
Mangin mirrors have classically been used with spherical mirrors to correct spherical aberrations. See, for example, the indicated disclosures within the following U.S. Pat. Nos.:
2,730,013, Mandler, Jan. 10, 1956, Col. 1, lines 19 and 46. PA1 2,817,270, Mandler, Dec. 24, 1957, Col. 1, lines 51-56 and 60-61. PA1 3,064,526, Lindsay, Nov. 20, 1962, Col. 5, lines 20-23 and 66-67. PA1 3,296,443, Argyle, Jan. 3, 1967, Col. 2, lines 33-36. PA1 3,632,190, Shimizu, Jan. 4, 1972, Col. 2, lines 34-39.
See also Rogers, "A Comparison Between Optimized Spheric and Aspheric Optical Systems for the Thermal Infrared", SPIE Vol. 147, Computer-Aided Optical Design, 1978, pp. 141-148 (text at bottom of paper 145).
(In addition, see U.S. Pat. No. 3,527,526, Silvertooth, Sept. 8, 1970, Col. 4, lines 64-67 which discloses use of a mangin in a cassegrainian arrangement for unspecified reasons).
While some of the above patents indicate use of a mangin mirror for correcting other aberrations, it is believed unique to configure mangin mirror surfaces for providing control of field curvature. As previously indicated, the field curvature problem becomes more severe as the optical system is made more compact. However, inasmuch as a mangin mirror may also be defined as a lens having its back surface coated with reflective material, it can be uniquely applied to correct the amount of field curvature by controlling the particular shape or radii of mangin mirror surfaces. Thus, for any given objective system or subsystem, the particular field curvature correction may be established by a unique set of values for the two radii of the mangin mirror surfaces. Further, this can be accomplished as an integral part of one of the two mirrors in a two mirror system and, therefore, does not require supplementing such systems with additional corrector lenses.